Add-ons allow developers to extend and modify the functionality of Firefox. They are written using standard Web technologies - JavaScript, HTML, and CSS - plus some dedicated JavaScript APIs. Among other things, an add-on could:

Change the appearance or content of particular websites

Modify the Firefox user interface

Add new features to Firefox

Developing add-ons

There are currently several toolsets for developing Firefox add-ons, but WebExtensions will become the standard by the end of 2017. The other toolsets, including overlay add-ons, bootstrapped add-ons, and the Add-on SDK, are expected to be deprecated over the same period of time.

If you are writing a new add-on, we recommend that you write a WebExtension.

WebExtensions are designed to be cross-browser compatible: WebExtensions written for Firefox will in most cases run in Chrome, Edge, and Opera with few if any changes. They are also fully compatible with multiprocess Firefox.

Most of the WebExtensions APIs are also available on Firefox for Android.

Migrate an Existing Add-on

If you maintain a legacy add-on, such as a XUL overlay, bootstrapped, or Add-on SDK-based add-on, we recommend that you investigate porting it to WebExtensions. There are some porting resources on MDN.

We've collected resources on a wiki page to support developers through the transition. To get started, use the compatibility Lookup Tool to see if your add-on will be affected.

Publishing add-ons

Addons.mozilla.org, commonly known as "AMO," is Mozilla's official site for developers to list add-ons, and for users to discover them. By uploading your add-on to AMO, you can participate in our community of users and creators, and find an audience for your add-on.

You are not required to list your add-on on AMO, but your add-on must be signed by Mozilla or users won't be able to install it.

Mobile add-ons are add-ons for Firefox for Android. Note, though, that we intend to deprecate some of the technology underlying these APIs. In the future, WebExtensions will be fully supported on Firefox for Android.